Many seismic investigation techniques have been developed for the purpose of ascertaining the nature of formations located in the vicinity of an open borehole.
One technique which has achieved widespread acceptance is that of acoustic well logging. Such logging generally utilizes an acoustic logging tool having a transmitter and receiver mounted thereon up to about 15-20 feet apart. This logging tool is moved through the borehole at a pre-selected speed while the transmitter is periodically activated. The transmission of waves through surrounding formations is then determined by recording and analyzing the acoustic energy received at the logging tool receiver. Generally, such data are analyzed to determine the velocity of at least the compression wave traveling through the adjacent formation to determine the nature of that formation. Attempts are also often made to determine the velocity of the shear wave and tube waves which travel in or in the vicinity of the wellbore. Such velocities are then utilized to identify formations disposed adjacent to the borehole.
Various techniques have been suggested for resonance measurements in geologic materials. In addition to the methods described in the above-identified, related patent applications, it is known to measure the resonant frequency of such materials using a bar resonance technique. Another quantity associated with resonance is the loss factor "Q". In a frequency vs. amplitude plot, where f.sub.0 is the frequency at which amplitude is at its maximum (A.sub.0), let amplitudes A.sub.1 and A.sub.2 equal A.sub.0 divided by the square root of 2. Amplitudes A.sub.1 and A.sub.2 will occur at frequencies f.sub.2 and f.sub.1 which are respectively greater and lesser than f.sub.0. The loss factor "Q" is determined by the formula: EQU Q=f.sub.0 /(f.sub.2 -f.sub.1)
Gardner, et al have reported various Q values for Berea materials by using a bar resonance method and frequencies of the order of 10 kHz. See Journal of Petroleum Technology, February, 1964, page 189.
Although various methods have been suggested for determining the nature of fluid content contained in reservoir formations, to date, none have been developed which are applicable to all kinds of formations under all conditions.